1. Field of the Invention
The present invention relates to powder coating compositions consisting of resinous polyhydroxyl compounds as film-forming agents, a compound comprising at least two functional groups reactive with the hydroxyl groups of said resinous polyhydroxyl compounds, as the cross-linking agent, and further comprising conventional additives, such as levelling agents, etc. and pigments, if so desired.
2. Description of the Prior Art
To produce powder coating compositions the components must be thoroughly mixed so that, following the mixing and powderizing treatments, the individual powder grains are of substantially the same composition which can only be achieved when the mixing is done in a heated extruder. Thereafter, the powder is spread onto the surface to be coated by way of the elctrostatic powder spray coating technique and baked at the required temperature for the respective coating composition. The preparation of powder coating compositions and the processing of such materials to lacquers requires that the individual components be miscible with each other at elevated temperatures and that the cross-linking reation take place only at baking temperatures. In practice, mixing temperatures of at least 90.degree. C, preferably between 100.degree. and 120.degree. C, have been found to be indispensible. The baking temperatures of powder coating compositions range without exception from 180.degree. to 200.degree. C with baking periods of from 20 to 30 minutes. Reduction of the baking temperatures to as low as 170.degree. C has been achieved by means of suitable activators.
Therefore, the powder coating compositions must include cross-linking agents that are not reactive with the hydroxyl groups of the polyhydroxyl compounds at mixing temperatures but which are reactive therewith at baking temperatures. This is achieved by employing cross-linking agents having functional groups protected (i.e., blocked) by compounds split off only at temperatures clearly ranging above said mixing temperatures. The most commonly used cross-linking agents today are diisocyanates or polyisocyanates like toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate; .omega.-lactams having from 4 to 12 carbon atoms, more especially .epsilon. caprolactam, proved to be suitable blocking agents. These blocked cross-linking agents, however, suffer from several disadvantages. For example, they escape while baking said lacquers which means not only establishing suitable hood means but also involves a considerable loss in material since the blocking agents have a relatively high molecular weight compared to the equivalent weight of the --N.dbd.C.dbd.O group to be blocked. Even though the baking temperature must range clearly above the melting temperature for achieving good processing prior to the cross-linking, the baking temperature should be the lowest possible for reasons of energy costs and in order that the substrate will be subjected only to low thermal stresses.
It is an object of the present invention to provide powder coating compositions that contain an unblocked cross-linking agent in which all components thereof are miscible with each other in the extruder at temperatures above 90.degree. C without any cross-linking reaction taking place, and which can be baked at the lowest possible temperatures.